Intraoperative control of intraocular infusion pressure is an important parameter in eye surgery. Liquid pressure regulation has been accomplished in most part using gravity-fed systems involving the relative height of the infusion bottle above the eye. A discussion of the development of gas infusion may be found in "Vitreous Microsurgery" by Steven Charles, M.D. in Williams & Wilkins, 1981, volume 4. As noted therein, there are known power injectors or pumps which are capable of maintaining an accurate intraocular pressure during air infusion, as compared to manual syringe injection. Such devices have also been developed wherein a microcompressor is used to produce an inflow of gas dependent upon intraocular pressure.
As these advances are made in gas infusion apparatus, a need exists for controlled intraocular infusion method and apparatus which would facilitate the interchangeability of gas infusion and liquid infusion during surgery on the posterior segment of the eye (vitrectomy). Additionally, during surgery on the anterior chamber of the eye (cataract extraction) the need exists for more accurate, surgeon controlled and monitored liquid infusion.
During cataract surgery, the most common operation performed in the United States, the surgeon views only the anterior chamber of the eye as shown in FIG. 5, having no method of simultaneously monitoring the posterior segment.
Since the anterior chamber is quite small, containing only 0.25 cc of liquid volume, small and momentary aspiration flow rates exceeding infusion rates will result in anterior chamber collapse, as shown in FIG. 5. The consequence of anterior chamber collapse is damage to the nonreproducible monolayer of cells (endothelium) which lines the inner surface of the cornea, keeping it clear of fluid. This endothelial damage can then result in clouding of the cornea, with the need for subsequent cornea transplantation.
It is common for cataract surgeons to perform incomplete temporary closure of the surgical incision prior to irrigation/aspiration cataract cortex removal. A true "closed-eye" system is never achieved, and anterior chamber collapse in this condition is a sign of wound leakage of infusion liquids rather than of inadequate infusion pressure. If anterior chamber collapse is encountered, the wound should be checked for tightness.
The cataract surgeon's most common defense against perceived anterior collapse is increasing of infusion pressure by raising a gravity-feed infusion bottle an estimated height above the eye as shown i n FIG. 4. Note that gas is not commonly infused into the anterior chamber in such surgery. In practice, bottle height is not measured, so that actual pressure delivered to the eye is unknown, and is presumed to be sufficient when anterior chamber collapse no longer occurs.
Unfortunately, corneal endothelial damage may also occur as a result of high infusate volume or jetstream mechanical damage from the use of high flow rates under high pressure. These effects, moreover, are not readily apparent to the surgeon due to his inability to perceive the flow rate or pressure. Retinal artery occlusion may also occur, invisible to the surgeon and resulting in blindness. Finally, iris prolapse through the wound may occur as a result of excessive infusion pressure.
Ideally, cataract irrigation/aspiration surgery should be performed with normal ocular pressure (25 mm Hg). Failing this, infusion pressure should be raised as little as possible to maintain anterior chamber pressure to avoid collapse during active irrigation/aspiration. Because unnecessarily high infusion pressure and flow rates can injure retinal and corneal tissues with little warning to the surgeon, increasing infusion to the eye should be the last solution attempted to remedy anterior chamber collapse. As previously stated, the surgeon should first check adequate wound closure. Further, even with an adequately closed eye, anterior chamber collapse may occur as a result of undisciplined, continuous, high rate aspirations. High vacuum aspiration is necessary to achieve cataract removal, but successful removal of blocks of cataract cortex material (with subsequent opening of the aspiration port) should be anticipated so as to intermittently reduce the aspiration vacuum, avoiding anterior chamber collapse. All too commonly surgeons move about within the eye with an open aspiration port, with suction continuously engaged.
The above discussion demonstrates the importance of a new method of rapid, accurate, surgeon-controlled ocular infusion (Gas Forced Liquid Infusion, GFLI).